1. Field of the Invention
The present invention relates to a communication method and system. For example, the present invention relates to an error correcting method and device for use with burst transmission in which data frames are successively transmitted in the field of communication.
2. Description of the Related Art
In burst communication, communication data are transmitted successively in the form of frames or packets. Hereinafter, the frame or packet is referred to as a frame. In burst communication, a frame may not arrive at a receiver (this is called nonarrival) or an error or errors may occur in data during transmission. To detect or recover nonarrival or errors, therefore, an error detecting or error correcting code can be used. To notify the transmitting end of the nonarrival or the occurrence of errors, the receiver can return an acknowledgment (ACK) to the sender.
The acknowledgment is made as follows: The sender sends a frame and the receiver then receives the frame and checks its contents. At this point, when an error detecting or error correcting code has been applied, it is decoded to determine whether the received frame is error-free. The receiver then sends an ACK (acknowledgment) frame to the sender. In the event of nonarrival or an error, the receiver sends no ACK frame. If the sender does not receive the ACK frame within a specified period of time after transmission of the frame, the sender retransmits the frame again. This process is called retransmission.
If, when a large amount of data is to be transmitted, an acknowledgment is made for each frame, the time required to make acknowledgments will require a large percentage of the overall transmitting/receiving time, resulting in reduced communication efficiency. There also exists a communication system in which a plurality of frames is acknowledged collectively. For example, the wireless LAN standard 802.11e specifies block acknowledgment. The block acknowledgment system involves transmitting multiple frames in succession (this transmission is referred to as a burst transmission) and acknowledging the received frames in a block. Specifically, the transmitter sends four frames, for example, in succession and then sends a block ACK request frame BAReq. The receiver requested to make acknowledgment sends a block ACK frame BA according to error conditions of the successively received frames.
The receiver decodes the error detecting and/or error correcting codes applied in advance to the frames and then acknowledges each of the frames according to the decoding results. If, at this point, no error is detected or errors, if detected, can be corrected, an ACK frame is sent; otherwise, no acknowledgment is made. There is also a communication system in which, to notify the sender of nonarrival or an error, the receiver sends to the transmitter a frame to the effect that the data has an error or the error cannot be corrected as an alternative to making no acknowledgment.
Some error correcting and/or error detecting codes require a long decoding time. When an error correcting or error detecting code has been applied to the block ACK system, the receiver decodes the code to determine error conditions. After that, the receiver sends a block ACK (BA) frame. In this case, the receiver must send the BA frame within a set period of time after receipt of the last frame. That is, the receiver must finish decoding the error correcting or error detecting code within that set period of time. It is therefore difficult to apply to the block ACK system an error correcting or error detecting code the decoding of which cannot be completed within the set period of time to the block ACK system.
In recent years, real-time contents, such as video and audio, are often transmitted over a communication link. In the event that some of the transmitted frames do not arrive or errors which cannot be corrected with an error correcting code are present, a problem arises in that the contents cannot be normally reproduced at proper times.
Japanese Patent Application Publication (KOKAI) No. 2004-80576 discloses a system which switches the modulation modes according to the position of PDU (Protocol Data Unit) within a frame.